A jack of this type can be used, for example, for the erection of an initially horizontal load into a more or less vertical position, as described in my commonly owned copending application Ser. No. 334,460 filed Dec. 28, 1981. If the load is to be maintained for an extended period in its erected position, it must be sustained by the jack after the oil supply has been cut off. The oil trapped inside the cylinder will prevent any untimely descent of the load only as long as there is no leakage. If the risk of such leakage cannot be excluded, a backsliding of the jack must be resisted by mechanical locking means.
Such locking means have heretofore taken the form of screws axially threaded into the cylinder for backstopping the piston in the position last reached. This operation is tedious and time-consuming, as is the subsequent unthreading of the screws when the jack is to be restored to its normal position. There is also the danger of human error which may overstrain the hydraulic system if an operator causes oil to pressurize the piston in the reverse direction before withdrawing the backstopping screws.
The problem is particularly severe when, as described in my copending application referred to above, the load must sometimes be moved past its position of unstable equilibrium so that the double-acting piston is to be mechanically locked against untimely forward as well as reverse displacement.
The delay involved in emplacing and removing such mechanical stops is costly in industrial applications and may be particularly harmful in military installations, as where the load to be erected is a missile; see, for example, commonly owned U.S. application Ser. No. 342,392 filed Jan. 25, 1982 by Antonio Tripoli et al, now U.S. Pat. No. 4,415,304.